A network system architecture enables multiple applications like voice, data and video to be mixed over the network that comprises switching nodes and communications links. Networks for these applications generally have been designed for a particular type of application, such as teleconferencing applications. Different types of applications, however, have diverse requirements.
“Quality of Service” or “QoS” parameters in part define these requirements. In the case of ATM networks, such parameters may include errored cell ratio (ECR), cell loss ratio, fixed delay and delay variation parameters. Applications may also indicate a set of parameters called a “traffic contract” that relates to the bandwidth required from the network. The Errored Cell Ratio (ECR) value is the ratio of the ATM cells in error to all the ATM cells sent during some interval. The remaining parameters are described later. Each application may have a maximum or minimum acceptable value for each parameter and a requested value for each parameter.
As previously indicated, networks are implemented with switching nodes and communications links. The communications links typically have been based upon conventional telephony transmission systems and comprised fiber optic, microwave or wireline links. Fiber optic links transfer typically an error rate of 10.sup.-9; microwave and wireline connections, 10.sup.-7. More recently, communications links have begun to comprise terrestrial and satellite mobile wireless communications links and cable television systems. Each of these communications links are prone to introducing errors at significantly greater rates.
Some present ATM networks attempt to overcome such errors by implementing a transformation process in the form of a correction, or data manipulation, technique at each switching node on a “per-communications link” basis. That is, the network is designed such that certain links operate with a transforming process that is not alterable on a dynamic basis. For example, it is fairly common that satellite systems have a single error correcting code that operates over the entire bit stream passing over the link, notwithstanding the specific application. Using error correcting codes, such as forward error correction (FEC) codes, requires significant amounts of redundant information to be sent with each block of bits wherein a “block” may comprise a partial ATM cell, a single ATM cell or a plurality of ATM cells. This redundant information adds “overhead” to the transfer. This, in turn, reduces the bandwidth available for transporting an ATM cell. To be effective, an error correcting code must also match the anticipated error burst characteristics of the transmission system; systems that are subject to longer bursts require larger codewords or interleaving or both. Interleaving adds significant amounts of fixed delay to that already present in the transmission system. Such fixed delays manifest themselves, for example, as awkward conversation and user discomfort in a teleconferencing application or even as motion sickness and disorientation in a virtual reality application.
Despite the foregoing efforts, network failures are inevitable, and there is a need of monitoring network performance for the purpose of maintaining a predetermined agreed upon QoS.